1. Field of the Invention
The invention relates to visibly transparent dyes and a method for using same in through-transmission laser welding. More particularly, it relates to the use of dyes in aesthetically demanding applications.
2. The Prior Art
Many different materials are used as brazing compounds or radiation scattering compounds in plastic welding. Generally the top substrate, through which the radiation is first transmitted before reaching the joint region, is designed to transmit a high portion of the welding radiation.
Brazing compounds are added to the joint region to absorb or scatter the incident radiation to melt the facing surfaces that comprise the joint to be welded. Radiation scattering is an uncontrolled and relatively inefficient method of converting the incident radiation to heat. Accordingly, radiation scattering may require larger amounts of the brazing compound, greater amounts of incident radiation, slower linear welding speeds, and may result in a poor appearance of the final weld.
Other radiation scattering or radiation adsorbing materials may be incorporated into the surface of either substrate at the joint region, or into the entire lower substrate. However, there are no known compounds or welding techniques which can provide predicable results in demanding aesthetic industrial and consumer applications like automobile moldings and food and beverage packaging. A brief description of those known materials and methods follows:
U.S. Pat. No. 4,424,435 employs metal brazing compounds, preferably titanium, but also silicon carbide, beryllium, cobalt, germanium, iron, molybdenum, nickel, niobium, platinum, rhenium, rhodium, tantalum and tungsten. Several severe limitations are inherent in the disclosed methods. First, these metallic brazing compounds are only suitable for welding glass. Second, the patent admits that some experiments with Nd-YAG pulsed lasers have resulted in some nonlinear effects at the weld joint which are not entirely understood.
U.S. Pat. No. 5,843,265 contemplates the use of inorganic materials, such as, but not limited to, pigments, fillers, fibers and reinforcing materials as radiation absorbing compounds. Their preferred embodiment employs a carbon black suspension which necessarily tints the host workpiece to a muddy grey or black. In addition, the use of a polychromatic, non-coherent radiation source requires the presence of cumbersome focusing elements and masks.
U.S. Pat. Nos. 4,156,626; 4,906,320; and 5,501,759 generically refer to printer""s ink, carbon black, and aniline dyes; opaque ink and other sufficiently opaque materials; and dark ink or dark polymeric film, respectively.
U.S. Pat. No. 5,893,959 refers to light scattering pigments or glass fibers. In order to hide the presence of the pigments, they prescribe tinting both workpieces to be black and opaque.
The published PCT application bearing International Application Number PCT/GB99/03241 discloses a host of organic dyes and metalated organic dyes such as cyanine dyes, squarylium dyes, croconium dyes, metal phthalocyanine dyes, metalated azo dyes and metalated indoaniline dyes.
The above survey indicates that there are a multitude of options for radiation absorbing compounds. An inordinate amount of experimentation, and expense associated with those rarer compounds, may be required to select a general purpose dye for aesthetically demanding applications. Further confusing the issue are numerous interrelated factors such as host selection and its polymerically related cousins, host compatibility, including solubility, the degree of visibility of the compound, the type and degree of tinting contributed by the dye, if visible, and the related issues of host thickness, and dye concentration as a function of host thickness, i.e. the concentration gradient.
It is an object of the invention to identify dyes having strong absorption bands above 350 nm.
It is an object of the invention to identify dyes that further have absorption troughs in the visible region.
It is an object of the invention to identify dyes that are also extremely soluble, add little or no light scattering and efficiently absorb laser radiation and via vibronic relaxation transmit heat to weld plastics.
It is an object of the invention to additionally quantify the relative optical densities of the absorption band and absorption troughs.
It is an object of the invention to identify dyes that further strongly transmit light across most or all of the visible spectrum.
It is an object of the invention to identify dyes that also have high photopic transmission values.
It is an object of the invention to provide quantitative means for identifying dyes that meet all of the above criteria.
These and other related objects are achieved according to the invention by providing selection criteria for dyes that predict efficiency and performance for plastics welding. A first stage quantitative calculation screens dyes that possess an absorption band that can be matched to a laser that is suitable for plastic welding. It also identifies absorption troughs in the visible spectrum and established relationships between the relative optical densities of the absorption band and trough.
A second stage quantitative calculation screens dyes for their contribution to the transparency of the substrate. By taking into consideration the results from both stages, the usefulness of a candidate dye can be quickly, easily and inexpensively determined. The key is identifying the most critical parameters that determine a dyes ability to perform in aesthetically demanding welding applications and subsequently discovering ways to rate the dyes and compare them to each other.